1. Field of the Invention
The present invention relates to blending of gases and, more particularly, to respiratory care and to a gas blender that mixes gases, monitors the mixed gases, and controls the mixed gases so that the gases are maintained at the desired levels, e.g., at a selected concentration content and gas pressure.
2. Background Art
Oxygen therapy is used to treat patients suffering from respiratory diseases. Many of these patients need higher fractions of inspired oxygen (F.sub.I O2) than exist in air (21% oxygen) to obtain sufficient oxygenation to their tissues. Oxygen therapy is of particular value in dealing with respiratory diseases resulting from inadequate ventilation and cases of hypoxemia, which is a relative deficiency of oxygen in the arterial blood. In clinical practice, arterial oxygen is quantified by measurement of the partial pressure exerted by the oxygen dissolved in plasma (PaO2). As a result, hypoxemia potentially affects the normal physiologic processes by increasing the cardiopulmonary work and impairing the maintenance of tissue oxygenation. Therefore, as research indicates, breathing a gas mixture with an increased F.sub.I O2 may correct the hypoxemia and reduce the respiratory work required to maintain a given PaO2.
Ventilation is the process of delivering oxygen to and washing carbon dioxide from the alveoli in the lungs. The movement of gas in the respiratory tract is tidal (to-and-fro) since the tract dead-ends in the alveoli and has only one outlet for gas to be exhaled from the system. Patients requiring oxygen therapy may also require mechanical ventilation due to respiratory failure. In these instances, a variety of mechanical ventilators is available. Most modern ventilators allow the clinician to select and use several modes of inhalation either individually or in combination. These modes can be defined in three broad categories: spontaneous, assisted, or controlled. During spontaneous ventilation without other modes of ventilation, the patient breathes at his own pace, but other interventions may affect other parameters of ventilation including, the tidal volume and raising the baseline pressure above ambient to improve oxygenation.
In assisted ventilation, the patient initiates the inhalation by lowering the baseline pressure to varying degrees, and then the ventilator "assists" the patient in completing the breath by the application of positive pressure. During controlled ventilation, the patient is unable to breathe spontaneously or initiate a breath, and is therefore dependent on the ventilator for every breath. During spontaneous or assisted ventilation, the patient is required to "work" (to varying degrees) by using the respiratory muscles in order to breathe. During controlled ventilation no work is required of the patient. Modem ventilators are designed to minimize or control the work required of the patient in the spontaneous or assisted modes. The work required by the patient is generally referred to as work of breathing and can be measured and quantified in Joules/breath or Joules/L of ventilation.
One of the main goals in treating patients suffering from respiratory disorders is to reduce the respiratory work. Research indicates that a low resistance breathing system that allows high gas flow rates on demand during spontaneous inhalation usually meets the requirement. As mentioned above, many modern ventilators are designed to minimize respiratory work. However, many require the serial connection of a gas blender between the gas supplies and the ventilator to provide the proper gas mixture to obtain the selected F.sub.I O2. Failure to have a blender capable of meeting the demands of the patient can compromise the effectiveness of the ventilator and consequently the health of the patient. At present many ventilators have "integral" blenders which may or may not meet the demand of the patient. Other ventilators presently rely on mechanical "stand alone" blenders which have been shown to have limited ability to meet the demands of the patient. Desirable characteristics of air/oxygen blenders used to produce high flow on demand are (1) a high-flow output greater than 120 liters/minute and (2) supply an approximately 50 pound per square inch gauge ("psig") output pressure.
Oxygen therapy may also have an adverse effect, especially if the F.sub.I O2 is not carefully controlled to the needs of the patient. One example is "absorption atelectasis," in which increases in alveolar oxygen concentration due to excessive F.sub.I O2 results in a reduction of the alveolar partial pressure of nitrogen. In alveoli with reduced ventilation but good perfusion, the volume of oxygen removed by the blood may be greater than the volume of gas that enters with each tidal ventilation. In this case, reduction of nitrogen partial pressure may allow the alveolar volume to decrease below a critical level, resulting in partial or complete collapse of the alveolus. The higher the F.sub.I O2, the greater the degree of denitrogenation and the more likely the presence of absorption atelectasis.
Another adverse effect of oxygen therapy is oxygen toxicity. The inherent toxicity of oxygen to the tissue was demonstrated almost a century ago. Intracellular oxygen metabolism involves the serial reduction of oxygen to water, a process that involves the formation of highly reactive free radicals, superoxide molecules (H.sub.2 O.sub.2) and hydroxyl ions (OH). These free radicals are capable of unregulated reactions with organic molecules that can result in damage to cell membranes and mitochondria and inactivation of cytoplasmic and nuclear enzymes. Therefore, oxygen toxicity is a potential problem in patients of any age.
It should be clear that an oxygen blender that can precisely and predictably maintain a selected F.sub.I O2 is indispensable in the care of patients with pulmonary diseases and respiratory failure.
Unfortunately, most stand-alone and some integral medical gas blenders used to control F.sub.I O2 have serious drawbacks, such as no or inadequate monitoring and correction safeguards. Accordingly, use of these prior art blenders may result in adverse effects during oxygen therapy. At present, all stand-alone blenders are entirely pneumatic and do not generate or react to any electrical signals. Therefore, the aforementioned stand-alone blenders are difficult to interface with computer controlled ventilators or other electronic medical equipment that currently exist. The prior art pneumatic blenders also suffer from inadequate peak flow rates and output pressure dropout.
Another problem is that the prior art blenders are dedicated to only two gases. That is, the blender's components are configured to accept only air and oxygen. For example, if helium and oxygen need to be mixed, tedious re-calibration and replacement of the oxygen/air dial by a homemade oxygen/helium dial is required.
The Bird blender is probably the best known prior art blender and, like other prior art devices, is pneumatic. The blender uses a balance module and a proportioning module to provide mixing of air and oxygen. The nominally 50 psig air and oxygen gas sources (usually 40-60 psig) enter through the respective inlet connectors. Each connector incorporates a filter to trap impurities. From the filter, the gases travel through a duckbill check valve which prevents possible reverse flow from either the air or oxygen supply systems. The two gases next enter a balance module to equalize the operating pressure of the air and oxygen gases before entering the proportioning module.
The gases then each flow into the proportioning module and mix according to the oxygen percentage selected by the control knob. This module consists of a double ended needle valve positioned between two valve seats. Of the two valve seats, one valve seat controls the passage of air and one valve seat controls the passage of oxygen into the blender outlet. At the outlet, the two gases have been blended according to the oxygen percentage selected on the control knob. With the blender control knob at the 21% oxygen position, the double ended needle valve will completely block the flow of oxygen allowing only air to flow. By adjusting the control knob at the 100% oxygen position, the flow of air is blocked permitting only the flow of oxygen to the gas outlets of the blender.
There are two gas outlets in the Bird blender: a primary outlet and an auxiliary outlet. The primary gas outlet is used for unmetered high flow applications in the range of 15-120 liters/minute The auxiliary outlet is designated to deliver metered gas through a flowmeter. Mixed gas may accurately be delivered from this outlet at 2 liters/minute and above. With the auxiliary outlet operational, there is a minimal bleed flow (10-12 liters/minute) from this outlet.
The Bird blender has other significant limitations. Low peak flow rates is one. As previously mentioned, a primary goal in treating patients suffering from respiratory disorders is to reduce the respiratory work. Thus, it is desirable for a low resistance breathing system to provide high gas flow rates on demand during spontaneous inhalation. Research indicates that a high flow output greater than 150 liters/minute is desirable from an air/oxygen blender. It appears that supporting patients with a high flow demand system for spontaneous breathing reduces inspiratory effort and therefore diminishes work of breathing. This effect is likely attributable to the fact that some patients may have instantaneous flow demands up to 200 liters/minute. A ventilator with a peak spontaneous flow rate capability of 80-100 liters/minute may not meet some patients' spontaneous flow rate requirements. Such patients may become agitated, diaphoretic (perspire), and "fight the ventilator" in an effort to achieve a sufficient flow demand. The selection of 150 liters/minute as a desirable minimum peak flow rate for a blender is based on clinical observations that low flow rates can be insufficient, but 150 liters/minute is almost always adequate. The Bird blender can deliver a maximum flow rate of 120 liters/minute only momentarily under ideal conditions of 60% F.sub.I O2 (1:1 mixture of air and oxygen) and 55 psig gas supply pressures. Peak flow rates of 90-100 liters/minute are achievable at other F.sub.I O2 settings.
The Bird blender has an open loop design and also lacks any means of self-monitoring. Without a built-in oxygen sensor, the blender cannot correct itself, or even provide an alarm, if delivered F.sub.I O2 is significantly different from the set F.sub.I O2.
The constant bleed flow of 12 liters/minute from the auxiliary outlet is wasteful. The auxiliary outlet is used for low flow rate applications requiring metered flow. When a flowmeter is connected to the auxiliary outlet, a constant flow of gas (10-12 liters/minute) bleeds from the main outlet of the blender to atmosphere and is lost.
Thus, there is a need in the art for a blender that overcomes the problems that exist with the prior art devices. The blender should be electric or otherwise compatible with other equipment instead of being entirely pneumatic.
A need also exists in the art for a blender that is self-monitoring for oxygen content and, if desired, gas pressure. This monitoring is essential because of the potential adverse consequences that may arise if the gases delivered to the patient are not carefully controlled. An associated need in the art is for a feedback system to correct any variations that exists between the desired/set and actual gas content and pressure.
Still another need in the art is for a blender that provides better flow control. It is also desired that the blender be designed to provide high peak flow rates for those applicable situations.
Yet another need in the art is for a blender that can use gases other than oxygen and air with out requiring a major reconfiguration of the blender components.